Knee osteoarthritis (KOA) is a very common degenerative joint disease in the elderly (Sharma, 2021), the most common cause of mobility limitation in adults (Katz et al., 2021). Pain and functional limitations caused by KOA negatively affect the quality of daily life (Felson et al., 2000). With the aging of the population, the prevalence of KOA is getting higher and higher, which brings a huge economic burden to both individuals and society (Cowan et al., 2015; Cui et al., 2020). Osteoarthritis (OA) has long been visualized as a “wear and tear” disease, but modern medicine is more aware of the complex pathophysiology that affects multiple joints and periarticular structures (Katz et al., 2021). KOA is primarily driven by a combination of biomechanical and inflammatory alterations that cause changes in the joints and surrounding muscles (Griffin and Guilak, 2005; Briggs-Price et al., 2022). As the “power and control device” of joint activity, muscle is a link that can not be ignored in the study of the occurrence and development of KOA (Culvenor et al., 2017; Zeng et al., 2022). Weakened muscle strength is closely related to structural degeneration and symptom development in KOA, even the muscle lesions appear earlier than the joints (Thomas et al., 2010).

As the largest and strongest muscle group of the lower extremities, the quadriceps femoris (QF) is an important part of the knee extensor mechanism. The QF consists of four muscles:rectus femoris (RF), vastus intermedius (VI), vastus medialis (VM) and vastus lateralis (VL). As the stabilizer and shock absorber of the knee, QF can dissipate the harmful load on the knee to a certain extent and help to maintain the mechanical environment of the joint (Øiestad et al., 2015). Previous studies have shown that QF strength is decreased and muscle cross-sectional area is reduced in KOA patients (Aslan et al., 2020; Yamauchi et al., 2020). In patients with knee osteoarthritis, changes in muscle strength and shape may be affected by many factors. Such as pain (Ruhdorfer et al., 2014), changes in muscle structure (Mohajer B), decreased physical activity, changes in joint load, obesity, poor alignment, trauma and joint instability. Therefore, it seems necessary to determine the current fixed muscle mass to assess muscle quality. Ultrasound (US) can clearly distinguish muscles, fat, bone and other structures. It has been reported that it can be applied to the evaluation of quadriceps femoris muscle, which is cheap, practical and safe (Noorkoiv et al., 2010). Measurement the muscle layer thickness by US is considered one of the methods to assess the muscle mass of QF (Koca et al., 2014).

Shear wave elastography (SWE) is a novel ultrasonography techniqie. It transmits and tracks shear waves to obtain Shear modulus values, and dynamically depicts the distribution of SWE in target tissues in real time (Niu et al., 2022). Shear modulus value can be used to quantify local muscle stiffness (Chalchat et al., 2022), and tiffness is a proxy for the force of muscle contraction (Lin et al., 2022). Evaluating the passive and active mechanical properties of QF can reflect the contractile performance of its muscles. Surface electromyography (sEMG) is widely used to assess muscle activation during isometric and dynamic actions of the limbs and trunk (Naik et al., 2017). RMS is an important index of sEMG, which refers to the integrated electromyography value divided by the time of measuring the integrated electromyography. It can reflect the effective value of nerve muscle fiber discharge, which is related to motor unit recruitment and excitatory rhythm synchronization (Pullman et al., 2000).

sEMG has been widely used to evaluate lower limb muscle function and coordination in KOA patients, and ultrasound has also been used to observe muscle structure to evaluate muscle mass and morphology. The use of Shear wave elastography to evaluate muscle biomechanical properties is still rare. This study was the first to comprehensively use the above equipments to evaluate the muscle quality, biomechanical properties and muscle contraction force of QF in patients with KOA, and to explore the correlation between the observed indicators. Based on the above equipment and indicators, we designed this study to comprehensively evaluate the muscle mass, biomechanical properties and muscle contractility of QF in KOA patients. In addition, we try to explore the differences of asymmetry between unilateral and bilateral patients in these characteristics, so as to provide some ideas for the prevention and treatment of KOA.

The occurrence and development of KOA have a serious impact on the muscle groups around the knee joint. After the muscle function is affected, the joint degeneration and muscle weakness of KOA patients will be further aggravated, and even muscle atrophy will occur, which is a vicious circle (Bennell et al., 2011; Bennell et al., 2013). And Thomas et al. found that the decrease of QF muscle abdominal cross-sectional area existed before the occurrence of KOA, which further confirmed the correlation between the decrease of muscle cross-sectional area and the incidence of KOA (Thomas et al., 2010). The main molecular mechanisms of muscle atrophy include decreased protein synthesis, increased breakdown, and impaired muscle satellite cell regeneration (Cohen et al., 2015). However, a study by Noehren et al. showed that the hypofunction of QF in KOA patients may not be caused by muscle fiber size damage, but by extracellular matrix expansion (Noehren et al., 2018). In addition, impaired satellite cell density, high profibrotic gene expression and Ⅰ-to-Ⅱ (slow-to-fast) fiber type switching may also contribute to the decreased muscle quality in KOA patients (Noehren et al., 2018). A study has found that compared with healthy people, only VM in QF showed significant decrease in muscle thickness in mild KOA patients, and other differences were not obvious (Taniguchi et al., 2015). However, the content of different types of muscle fibers may change, which has a negative impact on muscle quality and the cooperative contraction of quadriceps femoris is abnormal. Therefore, in the early and middle stages of KOA, it may not be comprehensive and accurate to judge muscle quality and function simply by muscle thickness or cross-sectional area.

In this study, the thickness of RF、VI、VM, and VL of RSL in UG was lower than those of RML. In BG, only the difference in the thickness of the VM between the two lower limbs was not significant, but the thickness of RF, VM, and VL was higher in RML. There was a significant reduction of QF thickness in RSL relative to RML, which may be related to more severe long-term pain, functional limitation, and load reduction (Tsukada et al., 2020). Several previous studies in human have suggested unilateral mechanical loading as a possible cause of muscle atrophy. In the model of single lower limb loading, the contralateral lower limb muscles would show disuse atrophy, and the longer the time was, the more serious the muscle reduction would be (Schulze et al., 2002; Al-Khlaifat et al., 2016). VM is the latest muscle to develop and the weakest muscle in quadriceps phylogenies. Therefore, disuse atrophy occurs first in VM after being affected by factors such as injury, immobilization, or surgery (Fox, 1975). Although the RML symptoms of BG patients are relatively mild, VM of BG patients may have different degrees of muscle atrophy, similar to RSL, because it is also the affected limb. The muscle thickness of RF, VI, and VL in RML may be significantly better than that in RSL because of the shorter course of disease and lower K/L grade. This is similar to the view of Taniguchi et al. (Taniguchi et al., 2015). In this study, there was no significant difference in asymmetry index of bilateral quadriceps thickness between BG and UG groups. This suggests that the quadriceps thickness asymmetry in BG is also evident. A study by Lee et al. used dual-energy X-ray absorptiometry to measure lower limb muscle mass in KOA patients. The asymmetry index of both lower limbs was then calculated (Lee et al., 2019). It was found that the asymmetry index was correlated with knee pain and significantly higher grade of radiographic KOA in male patients. However, there was no significant correlation in female patients. However, even in healthy elderly women, the asymmetry of lower limb muscles is more obvious than that of men (Lee et al., 2019; Mertz et al., 2019). It can be seen that the causes of this muscle mass asymmetry are complex and not only caused by unilateral or bilateral KOA. Therefore, training interventions should be used to enhance muscle strength and power to maintain and improve lower extremity function, while reducing asymmetric parameters is not so important (Mertz et al., 2019).

In order to conduct more accurate SWE assessment, RF, VM, and VL of the superficial layer of the QF were selected for measurement in this study. The SWE of the three muscles in the RML was higher than that in the RSL in both groups. In addition, the shear modulus asymmetry index of UG’s RF、VM, and VL were significantly higher than BG’s. Thus, it indicated that the leg with KOA was decreased by different degrees of muscle SWE compared with the healthy leg. It can be seen that the SWE of the muscles in the leg with KOA is lower than that in the healthy leg. The vicious cycle of “muscle spasm-pain- muscle spasm” is common in muscle diseases, which easily leads to increased muscle tone and stiffness (Bentley, 1996; van Dieën et al., 2003; Bialas et al., 2019; Maughan and Shirreffs, 2019). But the quadriceps muscle mass reduction and muscle weakness are the most common peripheral muscle disorders in patients with KOA (Slemenda et al., 1997). The pain mainly concentrated in the knee joint and around the tibial plateau, and the quadriceps femoris had relatively less pain in the early stage of KOA. Some studies have assessed the risk of muscle weakness and sarcopenia in COPD patients by measuring the RF SWE (Deng et al., 2021; Niu et al., 2022). The results showed that the shear modulus of patients was significantly lower than that of normal people, and the sensitivity of this index was higher than that of muscle thickness and cross-sectional area. The decrease in muscle stiffness could reflect muscle fiber atrophy (Wen et al., 2018) or a combination of muscle edema, inflammation or lipid accumulation, and fiber atrophy (Alfuraih et al., 2019). What’s more, SWE was independent of sex, height, and body mass, but muscle cross-sectional area and thickness are affected by height and weight (Deng et al., 2021). Therefore, SWE plays a role in evaluating the biomechanical property of QF and measuring muscle quality in KOA patients. Improving the SWE of quadriceps femoris muscle by intervention may have a positive effect on the prevention and treatment of KOA.

As expected, the RMS of the measured muscles in the RML was significantly lower in both groups during the sitting and leg raising task. In the squatting task, the RMS of muscles in the UG RSL was also significantly lower. While the three muscles in BG showed the same trend, but the differences were not significant. In bipedal tasks, the placement of body weight mostly through the unaffected or minor affected limb has been thought as one of the compensation strategy expected for KOA patients (Petrella et al., 2017). However, some studies evaluating balance function found that patients with bilateral KOA showed significant postural sway (Khalaj et al., 2014). Moreover, the adjustment strategy in the squatting task is complex (Hase et al., 2004), increasing the fear of pain (Vincent and Vincent, 2012) and the attempt to maintain postural control (Petrella et al., 2017). Thus, there may also be more passive activation of the lower limb muscles of the RSL in this unstable state. Eliminating inter-limbs sEMG asymmetry on the basis of improving the overall recruitment of motor units of both lower limbs may be beneficial for KOA. It can be noted that there is no significant difference in bilateral VI and VL in BP patients, but RMS of all three muscles in RML were significantly higher in the single-leg open kinetic chain task. It indicates that muscle thickness may not absolutely reflect muscle quality and function in already injured muscles. Combined with electrophysiological analysis of muscle activity, muscle contraction and activation ability can be analyzed in addition to muscle mass.

As mentioned above, the straight leg raising task in the sitting position may eliminate the interference of body instability and provide a relatively more accurate assessment of the actual muscle activation ability of the unilateral limb. The muscle sEMG asymmetry index in this task were significantly positively correlated with the degree of pain, and this was true in both groups. The UG muscle thickness asymmetry index is positively correlated with the degree of pain, which also supports that the more severe the symptoms of unilateral KOA patients, the affected limb may suffer from more serious muscle degeneration. In BG, only VM thickness asymmetry index is significantly positively correlated with the degree of pain, highlighting the initial and importance of VM degeneration in KOA, which needs to be paid attention to. The above asymmetry index may provide certain reference significance for the evaluation of the severity and treatment of KOA.

The muscle thickness, shear modulus and muscle activation electromyography of QF in RML were higher than those of RSL in unilateral KOA patients. The VM of RML in bilateral KOA patients may show muscle thickness degeneration earlier, which is closer to the VM of RSL. The shear modulus of RF, VM, and VL were higher on the RML side during the single-leg task, but there may be passive compensation for muscle activation in both lower limbs during the bipedal task. Our study found that the muscle thickness, shear modulus and muscle activation electromyography of QF in RML were higher than those of RSL in unilateral KOA patients. The VM of RML in bilateral KOA patients may show muscle thickness degeneration earlier, which is closer to the VM of RSL. The shear modulus of RF, VM, and VL were higher on the RML side during the single-leg task, but there may be passive compensation for muscle activation in both lower limbs during the bipedal task. This asymmetry in KOA patients found in our study may provide ideas for the assessment, treatment and rehabilitation of the disease, such as used for early evaluation of muscle parameters and formulation of corresponding muscle intervention plan guiding functional exercise.